An X-ray CT apparatus includes a pair of an involute gear and a magnetic sensor as a structure for detecting the rotation of the gantry rotary section in a non-contact fashion. The involute gear imparts a periodic change in the magnetic flux in the circumferential direction of a circle coaxial with the gantry rotary section. The magnetic sensor including the magnet and the magnetic reluctance (MR) elements detects the change in the magnetic flux caused by the rotation of the involute gear together with the gantry rotary section to generate a pulse signal conforming with the change in the magnetic flux. The pulse signal obtained from the magnetic sensor is supplied to the servo amplifier, the CT main control section, etc. via the signal processing unit.
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1. An X-ray CT apparatus, comprising:
a gantry including a substantially cylindrical rotary section and a stationary section rotatably holding the rotary section; rotation detecting means for detecting the rotation of the rotary section to generate a rotation detecting signal; correcting means for correcting the rotation detecting signal; and calculating means for calculating at least one of the rotating position and the rotating speed of the rotary section on the basis of the corrected detecting signal from said correcting means.
20. An X-ray CT apparatus comprising:
a gantry including a substantially cylindrical rotary section and a stationary section rotatably holding the rotary section; motor for directly driving the rotary section to rotate the rotary section; ring-shaped detectable section mounted on one of the rotary section and the stationary section and having a plurality of slits periodically arranged on said section; detecting means, mounted on the another of the rotary section and the stationary section, for detecting light rays passing through the slit to generate a rotation detecting signal; and means for generating an encoded signal based on the rotation detecting signal from said detecting means.
14. An X-ray CT apparatus, comprising:
a gantry including a substantially cylindrical rotary section and a stationary section rotatably holding the rotary section; rotation detecting means for detecting the rotation of the rotary section to generate a rotation detecting signal, said rotation detecting means including a ring-like target section to be detected, which is mounted to any one of said rotary section and said stationary section and having a periodic slit pattern formed therein, and an optical sensor mounted to the other of said rotary section and said stationary section in a manner to face said target section to detect the light ray passing through the slit pattern of the target section in accordance with rotation of the rotary section and, thus, to generate a detection signal; correcting means for correcting the rotation detecting signal; and calculating means for calculating at least one of the rotating position and the rotating speed of the rotary section on the basis of a corrected detecting signal from said correcting means.
18. An X-ray CT apparatus comprising:
a gantry including a substantially cylindrical rotary section and a stationary section rotatably holding the rotary section; detectable means, mounted on one of the rotary section and the stationary section, for applying a periodical change on a magnetic flux generated in a circumferential space which is coaxially defined around said rotary section; detecting means, mounted on the another of the rotary section and the stationary section, for detecting the change of the magnetic flux which is produced due to the rotation of the rotary section, to generate a rotation detecting signal; and calculating means for calculating at least one of the rotating position and the rotating speed of the rotary section on the basis of the detecting signal from said detecting means, wherein said detectable means includes a gear having tooth tips, and wherein said detecting means includes means for applying the magnetic flux in the circumferential space, and a magneto-resistance element for producing a change in the resistance corresponding to the change in said magnetic flux to generate an electric signal.
10. An X-ray CT apparatus, comprising:
a gantry including a substantially cylindrical rotary section and a stationary section rotatably holding the rotary section; rotation detecting means for detecting the rotation of the rotary section to generate a rotation detecting signal, said rotation detecting means including a target section to be detected, which is mounted to one of said rotary section and said stationary section to impart a periodic change in the magnetic flux in the rotating direction of the rotary section, and a rotation detecting sensor mounted to the other of the rotary section and the stationary section with a gap provided between said target section to detect the change in the magnetic flux generated in the target section in accordance with rotation of the rotary section and to generate a rotation detecting signal in accordance with detection of the change in the magnetic flux; a sensor for detecting a reference position of the rotary section to generate a reference position detecting signal; calculating means for calculating at least one of the rotating position and the rotating speed of the rotary section on the basis of said detection signal, said calculating means calculating the rotating position of the rotary section on the basis of the reference position signal generated from a position sensor and the rotating speed of the rotary section and also calculating a correction amount conforming with the position from said rotation detecting signal; and moving means for slightly moving said rotation detecting sensor relative to said target section in accordance with said correcting amount to maintain constant the gap between the target section and the rotation detecting sensor.
2. The X-ray CT apparatus according to
3. The X-ray CT apparatus according to
said control section includes an arithmetic calculating section for calculating the rotating position of said rotary section based on the reference position signal generated from said sensor and the rotating speed of said rotary section.
4. The X-ray CT apparatus according to
5. The X-ray CT apparatus according to
said calculating means includes an arithmetic section for calculating the rotating position of said rotary section based on the reference position signal generated from said position sensor and the rotating speed of said rotary section; and said arithmetic section specifies the position of the connection portion of said slit plate, calculates the correcting value for correcting the rotation detecting signal generated from the specified position, and corrects the rotation detecting signal with the correcting value.
6. The X-ray CT apparatus according to
a target section to be detected, which is mounted to any one of said rotary section and said stationary section to impart a periodic change in the magnetic flux in the circumferential direction of a circle coaxial with said rotary section; and rotation detecting means mounted to the other of the rotary section and the stationary section for detecting the change in the magnetic flux that is generated in said target section in accordance with rotation of said rotary section to generate an electric signal conforming with said change in the magnetic flux.
7. The X-ray CT apparatus according to
8. The X-ray CT apparatus according to
9. The X-ray CT apparatus according to
11. The X-ray CT apparatus according to
12. The X-ray CT apparatus according to
13. The X-ray CT apparatus according to
15. The X-ray CT apparatus according to
16. The X-ray CT apparatus according to
17. The X-ray CT apparatus according to
19. The X-ray CT apparatus according to
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This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-187874, filed Jun. 22, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to an X-ray CT (computed tomography) apparatus, particularly, to a mechanism for detecting the rotation of a rotary section of a gantry included in the X-ray CT (computed tomography) apparatus and to the control of the detecting mechanism.
In recent years, the function of the X-ray CT apparatus has been enhanced and diversified. One of the diversifications of the function is small sound imaging, i.e., the situation in which mechanical vibration is suppressed as much as possible so as to photograph a patient to be examined in a quiet operating environment. In a system in which the rotary driving force generated from a motor arranged in the stationary section of the gantry is transmitted via a gear, a belt, etc. so as to drive the rotary section, a big operating sound (noise) is generated by, for example, the sliding of the belt so as to produce anxiety or unpleasantness in the patient or the X-ray engineer. In these circumstances, the sound suppressing technology is applied to the rotary driving mechanism of the X-ray CT apparatus in order to prevent such a noise.
One of the technologies that has appeared in accordance with the trend toward small sound imaging is an X-ray CT apparatus in which a direct drive (DD) motor driving system is applied to the rotary mechanism of the gantry. In the direct drive motor driving system, a large number of magnets are mounted to the rotary section of the gantry. At the same time, a winding is mounted to the stationary section of the gantry. By supplying an electric current to the winding, the rotary section of the gantry is directly rotated as a rotor of the motor. The mechanism for transmitting the rotating force such as a belt and a gear is not included in the direct drive motor driving system, making it possible to suppress the sound.
A resolver mechanism is mounted to the X-ray CT apparatus employing such a direct drive motor driving system in order to detect the rotation of the rotary section of the gantry and to control the rotating position and the rotation speed.
The resolver 300 applied to the X-ray CT apparatus, which is used for detecting the rotating position of the gantry rotary section and for detecting the rotating speed or rate, is shaped like a ring having a relatively large diameter and conforming with the shape of the gantry rotary section. A reference signal consisting of a Sin wave is generated from a reference signal generator 305 for the resolver, which is mounted in the rotary section of the gantry, so as to be supplied to the resolver 300. Upon receipt of the reference signal, the resolver 300 generates a sin signal and cos signal, and these signals are supplied to a controller 306 mounted in the stationary section of the gantry. These sin signal, cos signal and the reference signal generated from the reference signal generator 305 through the transformer 301 are converted into a digital signal by an R/D circuit (reference/detection signal processing circuit) so as to form the basic pulse. The frequency of the digital signal is divided so as to be utilized for detecting the position of the rotary section. To be more specific, the basic pulse is supplied to the control section of the X-ray CT apparatus so as to be utilized for various control operations based on, for example, the detection of the rotary position and the detection of the rotating speed, thereby controlling the explosion of the X-ray, the data collection and the trigger of an X-ray tube.
The rotary detection mechanism utilizing the resolver described above gives rise to problems as described below.
First of all, the resolver is a structure having a large diameter. For the mass production of the resolver, large scale automation facilities are required, leading to a high manufacturing cost of the X-ray CT apparatus. Also, in the conventional gantry rotary section having the resolver mechanism incorporated therein, a large resolver structure, etc. is arranged behind a data transfer unit for transmitting data by optical communication utilizing an LED or a photo diode between the gantry stationary section and the rotary section. As a result, it is difficult to gain access to the resolver mechanism in performing, for example, a maintenance operation, giving rise to the problem that much labor is required for the inspection, the renewal operation, etc. It should also be noted that a signal transmitting line is drawn directly from the coil of the resolver mechanism in order to take out signals from the coil of the resolver mechanism. What should be noted is that, in the event of, for example, wire breakage, it is difficult to cope with the wire breakage problem flexibly, with the result that it is unavoidable to renew the entire resolver.
An additional problem to be noted is that the resolver is incapable of executing the function of detecting the rotation unless the reference signal generator for generating the reference signal is mounted to the stationary section of the gantry as described above so as to supply power to the entire rotary section and stationary section of the gantry via a slip ring. This particular construction is undesirable in terms of efficiency and safety in the maintenance step. Under these circumstances, vigorous efforts are being made in an attempt to develop a new improvement capable of overcoming the problems inherent in the mechanism for detecting the rotation by a resolver.
An object of the present invention is to provide an X-ray CT apparatus simple in construction, permitting the maintenance operation at a low cost, and capable of detecting rotation and controlling the rotation with a high accuracy.
According to a first aspect of the present invention, there is provided an X-ray CT apparatus, comprising:
a gantry including a substantially cylindrical rotary section and a stationary section rotatably holding the rotary section;
rotation detecting means for detecting the rotation of the rotary section to generate a rotation detecting signal;
correcting means for correcting the rotation detecting signal; and
calculating means for calculating at least one of the rotating position and the rotating speed of the rotary section on the basis of the corrected detecting signal from the correcting means.
According to a second aspect of the present invention, there is provided an X-ray CT apparatus, comprising:
a gantry including a substantially cylindrical rotary section and a stationary section rotatably holding the rotary section;
rotation detecting means for detecting the rotation of the rotary section to generate a rotation detecting signal, the rotation detecting means including a target section to be detected, which is mounted to one of the rotary section and the stationary section to impart a periodic change in the magnetic flux in the rotating direction of the rotary section, and a rotation detecting sensor mounted to the other of the rotary section and the stationary section with a gap provided between the target section to detect the change in the magnetic flux generated in the target section in accordance with rotation of the rotary section and to generate a rotation detecting signal in accordance with detection of the change in the magnetic flux;
a sensor for detecting a reference position of the rotary section to generate a reference position detecting signal;
calculating means for calculating at least one of the rotating position and the rotating speed of the rotary section on the basis of the detection signal, the calculating means calculating the rotating position of the rotary section on the basis of the reference position signal generated from a position sensor and the rotating speed of the rotary section and also calculating a correction amount conforming with the position from the rotation detecting signal; and
moving means for slightly moving the rotation detecting sensor relative to the target section in accordance with the correcting amount to maintain constant the gap between the target section and the rotation detecting sensor.
Further, according to a third aspect of the present invention, there is provided an X-ray CT apparatus, comprising:
a gantry including a substantially cylindrical rotary section and a stationary section rotatably holding the rotary section;
rotation detecting means for detecting the rotation of the rotary section to generate a rotation detecting signal, the rotation detecting means including a ring-like target section to be detected, which is mounted to any one of the rotary section and the stationary section and having a periodic slit pattern formed therein, and an optical sensor mounted to the other of the rotary section and the stationary section in a manner to face the target section to detect the light ray passing through the slit pattern of the target section in accordance with rotation of the rotary section and, thus, to generate a detection signal;
correcting means for correcting the rotation detecting signal; and
calculating means for calculating at least one of the rotating position and the rotating speed of the rotary section on the basis of a corrected detecting signal from the correcting means.
Yet further, according to a fourth aspect of the present invention, there is provided an X-ray CT apparatus, comprising:
a gantry including a substantially cylindrical rotary section and a stationary section rotatably holding the rotary section;
detectable section, mounted on one of the rotary section and the stationary section, for applying a periodical change on a magnetic flux generated in a circumferential space which is coaxially defined around the rotary section;
detecting means, mounted on the another of the rotary section and the stationary section, for detecting the change of the magnetic flux which is produced due to the rotation of the rotary section, to generate a rotation detecting signal; and
calculating means for calculating at least one of the rotating position and the rotating speed of the rotary section on the basis of the detecting signal from the detecting means.
Furthermore, according to a fourth aspect of the present invention, there is provided an X-ray CT apparatus, comprising:
a gantry including a substantially cylindrical rotary section and a stationary section rotatably holding the rotary section;
motor for directly driving the rotary section to rotate the rotary section;
ring-shaped detectable section mounted on one of the rotary section and the stationary section and having a plurality of slits periodically arranged on the section;
detecting means, mounted on the another of the rotary section and the stationary section, for detecting light rays passing through the slit to generate a rotation detecting signal; and
means for generating an encoded signal based on the rotation detection signal from the detecting means.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.
X-ray CT apparatuses according to some embodiments of the present invention will now be described with reference to the accompanying drawings.
A reference numeral 1 shown in
In the system in which the rotary driving force generated from a motor mounted to the stationary section of the gantry is transmitted through a gear, a belt, etc. so as to drive the rotary section of the gantry, a big operating noise based on, for example, the sliding of the belt, i.e., a mechanical noise, is generated so as to impart an anxiety or unpleasantness to the patient and the X-ray engineer. In order to suppress the noise, a rotation driving mechanism to which is applied a noise suppressing technology described below is incorporated in the X-ray CT apparatus widely used nowadays.
Employed in the X-ray CT apparatus according to the first embodiment of the present invention is a direct drive (DD) motor driving system as a low noise type rotation driving system. To be more specific, a winding 11 is mounted to the gantry rotary section 1, and a magnet 12 is mounted to the gantry stationary section 4. If an electric current is supplied to the winding 11, the magnetic flux generated from the magnet 12 and the magnetic flux generated from the winding 11 are repelled each other, with the result that the gantry rotary section 1 is directly rotated in a non-contact fashion as a rotor of the motor. In the present invention, a direct drive (DD) motor driving system is employed in this embodiment as a low noise type rotation driving system. However, in the X-ray CT apparatus of the present invention, the driving system is not limited to the direct drive (DD) motor driving system. As apparent from the entire description of this specification, the driving system is not limited to a particular rotation driving system.
In the X-ray CT apparatus according to the first embodiment of the present invention, employed is the mechanism for detecting the rotation of the gantry rotary section 1 in a non-contact fashion as described below. Specifically, an involute gear 10 is mounted as an object of detection of the rotation on the cylinder outer circumferential surface of the gantry rotary section 1 facing the stationary section 4, as shown in FIG. 2. The involute gear 10 is rotated in the circumferential direction of a circle coaxial with the gantry rotary section 1, and a periodic change in the flux is generated in the gap between the stationary section 4 and the rotary section 1 in accordance with rotation of the involute gear 10. The involute gear 10 is advantageous in that the gear 10 can be manufactured easily with a low cost and that the processing accuracy is high. Incidentally, it is possible to apply, for example, a skiving treatment to the magnetic portion of the gantry rotary section 1 so as to have the involute gear 10 formed integrally with the magnetic portion.
On the other hand, a magnetic sensor 5 is mounted to the gantry stationary section 4 so as to be positioned close to the involute gear 10 of the gantry rotary section 1. The gap between the magnetic sensor 5 and the involute gear 10 is set at a predetermined length, e.g., at about 1 mm. The magnetic sensor 5 generates a detection signal having a wave form dependent on the change in the magnetic flux in accordance with rotation of the involute gear 10 together with the gantry rotary section 1, and the detection signal thus generated is processed so as to output a pulse signal conforming with the detection signal.
The pulse signal obtained by the magnetic sensor 5 is supplied to a servo amplifier 8, a CT main control section 9, etc. via a signal processing unit 6. A switch and a correcting circuit 7 are arranged within the signal processing unit 6. The signal processing unit 6 will be described herein later in detail. The signal from the servo amplifier 8 is supplied to a DC motor driving circuit 14 serving to supply a driving signal to the winding 11 of the DC motor.
The DC motor driving circuit 14 generates a driving signal in accordance with the output from the servo amplifier 8 so as to rotate the rotary section 1 at a rotating speed set by the CT main control section 9. The CT main control section 9 supplies an instruction to start or stop the rotation or an instruction to set the rotary mode including an instruction of the rotating speed to the DD motor driving circuit 14. A reference position sensor 16 is mounted to the stationary section 4 of the gantry. A certain reference position of the rotary section 1 is detected by the sensor 16, and a reference position signal is generated from the sensor 16 so as to be supplied to the CT main control section 9. It follows that, upon receipt of the reference position signal, the CT main control section 9 obtains by the arithmetic calculation the rotating position of the rotary section 1 on the basis of the time from the timing of receiving the reference position signal and the rotating speed of the rotary section 1 so as to generate a position signal. In other words, the servo amplifier 8 receives the pulse signal from the magnetic sensor 5 and controls the direct drive motor based on the pulse signal so as to control the rotation driving of the gantry rotary section 1. The CT main control section 9 also receives the pulse signal from the magnetic sensor 5 so as to calculate the rotating position and the rotating speed of the gantry rotary section 1 based on the pulse signal, thereby controlling the operation of the entire X-ray CT apparatus. Incidentally, in the practical mounting, the CT main control section 9 is said to require a higher accuracy than the servo amplifier 8 relative to the pulse signal generated from the magnetic sensor 5.
The principle of detecting the rotation of the rotary section 1 by the combination of the magnetic sensor and the involute gear will now be described with reference to
As apparent from
In the practical mounting, the number of teeth of the involute gear 10 is set at, for example, 432. In this case, 432 sine wave signals SA of phase A and 432 cosine wave signals SB of phase B are generated and subjected to an A/D conversion and the resultant digital signals are supplied to, for example, a 25 multiplying circuit, with the result that 10800 phase A pulse signals and phase B pulse signals are generated for one complete rotation of the involute gear 10.
In this embodiment of the present invention, some measures given below are take against the distortion of the wave form of the output signals from the magnetic sensor 5 in the circuit construction described above.
In the X-ray CT apparatus, a high accuracy is required for the output signal from the magnetic sensor 5 for performing the position detection and the rotation control of the gantry rotary section 1 with a high accuracy. To be more specific, the error of the pulse signals of phases A and B is required to fall within a predetermined range, e.g., a range of ±3%. If distortion is included in the wave form of the signal before the A/D conversion, the accuracy of the pulse signal multiplied by 25 exceeds the error range of ±15%.
Under the circumstances, first and second optimizing technologies are applied to the shape of the involute gear 10 that is to be detected by the magnetic sensor 5.
It should be noted that, even if the shape of the involute gear 10 is made optimum, there are cases where a desired accuracy is not achieved. To be more specific, there are cases where the magnetic flux density fails to be rendered uniform because of the shape of the magnet 51 constituting the magnetic sensor 5, the shapes of the magneto-resistance elements 52, 53 and the arrangement of these magnet 51 and the magneto-resistance elements 52, 53, with the result that the wave form of the output signal remains to be distorted.
Under the circumstances, the output signal generated from the magnetic sensor 5 is corrected by the correcting circuit 7 shown in
Then, the measured time, i.e., the count signal, is multiplied by 1/100 so as to form corrected pulse signals each having a pulse length that is ¼ of the 25 multiplied pulse referred to previously, and these corrected pulse signals are successively outputted starting with the pulse after completion of the calculation. In other words, the corrected pulses are outputted at a time interval of 1/100 of the time between the adjacent zero-cross points.
The signal correction in the correcting circuit 7 is performed on the premise of the calculation based on the time measurement. However, it is possible for the calculation for the correction not to be performed because of the restriction in respect of the calculation process capability in the stopping period, the rotation accelerating period and the rotation decelerating period of the gantry rotation section 1, i.e., because of the restriction that it is possible to mount a large scale buffer in the correcting circuit 7. Such being the situation, the correcting operation is performed during the period when the gantry rotary section 1 can be rotated at a constant speed including the maximum speed, excluding the accelerating period or the decelerating period when the rotation of the gantry rotary section 1 is accelerated or decelerated, as shown in FIG. 7. In other words, the CT main control section 9 controls the ON/OFF of the correcting operation performed by the correcting circuit 7 as shown in FIG. 2. By allowing the CT main control section 9 to perform the control of the correcting operation as described above, it is possible to obtain a pulse signal to which the correction is applied only when required.
Incidentally, the original signal to which the correction by the correcting circuit 7 is not applied in respect of the signal supplied to the servo amplifier 8 is supplied to the signal processing unit 6. Also, the original signal, which is not corrected, is supplied to the CT main control section 9 during the rotation accelerating period and the decelerating period of the gantry rotary section 1. This is because a stabilized number of pulse signals are required for the position detection performed by the CT main control section 9.
As described above, according to the first embodiment of the present invention, the mechanism utilizing the magnetic sensor 5 and the involute gear 10 is applied for detecting the rotation of the gantry rotary section 1, making it possible to achieve marked improvements in the miniaturization of the construction, the simplification and the space saving, compared with the conventional resolver system. For example, the resolver mechanism weighs about 80 to 100 kg, whereas the sum of the weights of the magnetic sensor 5 and the gear 10 is only about several kilograms. Also, the manufacturing cost of the rotation detecting mechanism of the present invention is about half the manufacturing cost of the resolver mechanism. If the rotation detecting mechanism of the present invention is miniaturized, the construction of the entire gantry rotary section 1 is rendered simple, leading to improvements in the assembling capability, the maintenance capability and the safety.
In the conventional gantry rotary section to which the resolver mechanism is applied, a large resolver structure, etc. are arranged behind the data transmitting unit for achieving the data transmission between the gantry stationary section and the rotary section by the optical communication utilizing an LED or a photodiode. As a result, it is difficult to gain access to the resolver mechanism in performing, for example, the maintenance operation, giving rise to the problem that a much labor is required for the inspection, the renewal operation, etc. In the rotation detecting mechanism according to the first embodiment of the present invention, however, it is possible to gain access easily to the magnetic sensor 5 or the involute gear 10, making it possible to achieve, for example, the renewal operation in the site of the installation. It is also possible to inspect the function relating to the rotation mechanism of the gantry rotary section 1 by simply supplying an electric current to the motor system and the gantry stationary section 4 without supplying an electric current to the gantry rotary section 1 so as to contribute to the improvement in the maintenance efficiency.
It should also be noted that the rotation detecting mechanism according to the first embodiment of the present invention makes it possible to resolve the problem of the accuracy generated by applying the conventional magnetic sensor.
What should also be noted is that the shape of the involute gear 10 is made optimum so as to stabilize the output pulse signal generated from the magnetic sensor 5 formed of the magneto-resistance elements 52, 53, etc. Also, the pulse signal generated from the magnetic sensor 5 is corrected by the correcting circuit 7. It follows that it is possible to realize a high accuracy required in an X-ray CT apparatus.
To be more specific, it is possible to improve the pulse accuracy, with the voltage fluctuation between 4.0 V and 6.0 V and the gap fluctuation of 1.5 mm to 0.5 mm between the gear and the magnetic sensor taken into account, from the conventional value of ±1 to 2% to ±0.0093%.
As described above, according to the first embodiment of the present invention, it is possible to provide an X-ray CT apparatus simple in construction, high in its maintenance capability, and capable of achieving the rotation detection and the rotation control with a high accuracy.
It is possible to reverse the arrangement of the magnetic sensor 5 and the involute gear 10 on the premise that the detection signal generated from the magnetic sensor 5 is transmitted appropriately between the gantry rotary section 1 that is rotated continuously and the stationary section 4. To be more specific, it is possible to mount the magnetic sensor 5 in the gantry rotary section 1 and to mount the involute gear 10 to the gantry stationary section 4.
It is also possible to use another gear such as a spur gear in place of the involute gear 10. In this case, it is desirable to change appropriately the shape optimization of the involute gear 10 described above in accordance with the shape and the characteristics of the gear. In short, it suffices to suppress the distortion in the wave form of the output signal caused by the shape or arrangement of the gear itself serving to impart a change in the magnetic flux in the vicinity of the magnetic sensor.
Needless to say, it is possible to apply the technical idea of the present invention to the X-ray CT apparatus in which the driving mechanism of the gantry rotary section differs from the direct drive (DD) driving system, e.g., an X-ray CT apparatus equipped with a mechanism for driving the gantry rotary section by transmitting the rotary driving force generated from a motor via a gear, a belt, etc. In this case, the rotation detection is of non-contact type, making it possible to obtain the prominent effect of the present invention described above, though the noise suppressing effect is somewhat lowered.
In the second embodiment of the present invention, the pulse accuracy is improved by controlling the movement of the magnetic sensor toward or away from the rotary section 1. The second embodiment can be combined with the first embodiment described previously or can be worked independently.
In the case of employing the mechanism in which the involute gear 10 is mounted to the gantry rotary section 1 of the X-ray CT apparatus and the rotation of the involute gear 10 is detected by the magnetic sensor 5 as in the first embodiment of the present invention, the involute gear 10 is rendered relatively large in its diameter. As a result, it is difficult in some cases to manufacture the involute gear 10 as an ideal part close to the correct circle.
If there is a distortion, a recess, a warping, etc. in the shape of the involute gear 10, the gap between the magnetic sensor 5 and the involute gear 10 is changed, with the result that the accuracy of the detection signal generated from the sensor 5 is affected by the shape of the gear 10. This is also the case with the mounting accuracy to the gantry rotary section 1 as well as the processing accuracy of the involute gear 10.
In the second embodiment of the present invention, the magnetic sensor 5 is controlled to be moved toward or away from the involute gear 10 so as to maintain constant the gap between the involute gear 10 and the magnetic sensor 5. In order to obtain the control amount for the particular control, the gantry rotary section 1 is rotated preliminarily and the control amount is determined by the rotating operation. To be more specific, the gantry rotary section 1 is rotated to make at least one complete rotation, and a pulse signal for at least one complete rotation of the rotary section 1 is generated from the magnetic sensor 5 so as to calculate the pulse accuracy for every rotating angle (position) from the pulse signal thus generated. For example, where a recess is generated in a part of the involute gear 10, the pulse signal generated from the magnetic sensor 5, which corresponds to the recessed portion, is fluctuated depending on the recess, i.e., the defect, and the defect data on the recessed portion is obtained as the control amount by the preliminary rotation, with the result that the defect is recognized in advance. The particular recognizing treatment (measurement of the pulse accuracy) is executed by, for example, the CT main control section 9 so as to be stored as the defect data in the memory of the CT main control section 9.
Then, the control amount for moving the magnetic sensor 5 is calculated on the basis of the defect data obtained by the recognizing treatment, and the mechanism shown in
It is possible to realize a further improvement in accuracy and stabilization by executing the control to move the magnetic sensor 5 toward and away from the involute gear 10 for measuring the pulse accuracy appropriately in accordance with the operating state of the X-ray CT apparatus.
For example, it is desirable for the measurement of the pulse accuracy by the preliminary rotating operation described above and for the control to move the magnetic sensor 5 toward or away from the involute gear 10 to be executed for every rotating speed of the gantry rotary section 1 so as to prepare the control data.
Another example is the application to the scanogram or scanoscope photographing. In the scanogram photographing, the X-ray tube is moved to a desired rotating position and a ceiling plate having a patient disposed thereon is moved while maintaining the rotating position of the X-ray tube. In the scanogram photographing of this type, it is possible for the X-ray tube not to be moved to the predetermined rotating position noted above with a high accuracy because of the nonuniformity of the gap between the involute gear 10 and the magnetic sensor 5. Such being the situation, the preliminary rotating operation described above is performed before the scanogram photographing so as to measure the pulse accuracy by preliminarily rotating the gantry rotary section 1 to make at least one complete rotation, and the control to move the magnetic sensor 5 toward or away from the involute gear 10 is performed, thereby moving the X-ray tube to the rotating position noted above with a high accuracy.
In the scanogram mode, the gantry is driven to make one complete rotation so as to detect the pulse accuracy for every rotating position, as shown in step S23. Based on the detected pulse accuracy, the control amount relating to the distance between the magnetic sensor 5 and the involute gear 10 for every rotating position is determined as shown in step S28, and the driving mechanism of the magnetic sensor 5 is controlled based on the control amount such that the pulse accuracy falls within a predetermined range no matter where the X-ray tube may be positioned, as shown in step S25. The relationship between the position and the control amount is collected as shown in step S26 and stored in the memory device (not shown) of the CT main control section 9, thereby finishing a series of operations as shown in step S27.
The X-ray CT apparatus according to the second embodiment of the present invention described above produces the function and effect similar to those produced by the apparatus according to the first embodiment described previously and permits further improving and stabilizing the accuracy of the rotation detection.
In an X-ray CT apparatus according to a third embodiment of the present invention, a slit disk 91 is mounted in place of the involute gear 10 to the rotary section 1, as shown in
The gantry of the X-ray CT apparatus has the construction of a direct drive (DD) motor driving system as in the first or second embodiment described previously. Incidentally, it is possible to employ the indirect driving system utilizing, for example, a belt in place of the DD motor driving system.
In the X-ray CT apparatus shown in
It is possible for the magnetic reluctance sensor 5 to be mounted to the slit disk 91 such that each slit 92 of the slit disk 91 successively crosses the magnetic reluctance elements 52 and 53 as shown in FIG. 3A. Alternatively, it is possible for the magnetic reluctance sensor 5 to be similar in construction to the photo interrupter 90 shown in
In the combination of the transmission photo sensor 90 and the slit disk 91, if the slit disk 91 is rotated in accordance with rotation of the gantry rotary section 1, the light ray generated from the light generating section 94 passes through the slit disk 91 while the slit pattern 92 crosses the optical path of the light ray. It follows that a pulse is generated from the detecting section 95 to conform with the passage of the light ray through a single slit. Where, for example, 5400 slits are formed in the slit disk 91, 5400 pulse signals are generated by one complete rotation of the slit disk 91, and these pulse signals are multiplied by two by the multiplier circuit (not shown) so as to obtain 10,800 pulse signals. These pulse signals depend on the rotation of the gantry rotary section 1 and are supplied to the servo amplifier 8, the CT main control section 9, etc. so as to be utilized for detection of the rotating position, for the detection of the rotating speed, etc. of the gantry rotary section 1 as in the first embodiment.
Similarly, in the combination of the magnetic reluctance sensor 5 and the slit disk 91, the magnetic flux generated from the magnet 51 is guided into the magnetic reluctance elements 52, 53 when the slit 92 of the slit disk 91 is arranged between the magnet 51 and the magnetic reluctance elements 52, 53 so as to increase the resistance of these elements 52, 53. On the other hand, when the bridge between the adjacent slits 92 is arranged between the magnet 51 and the magnetic reluctance elements 52, 53, the magnetic flux generated from the magnet 51 is inhibited by the bridge between the adjacent slits 92 and, thus, is not guided to the magnetic reluctance elements 52, 53 so as to lower the resistance of these elements 52, 53. It follows that detection signal that are periodically changed as described previously with reference to
The slit disk 91 shown in
Under the circumstances, the correcting data are collected as shown in
Incidentally, in the rotation detection by the optical system shown in
A fourth embodiment of the present invention relates to the non-contact type rotation detection using a simplified encoder.
The gantry of the X-ray CT apparatus employs the direct drive (DD) motor driving system like each of the first to third embodiments described above. Incidentally, it is possible to employ the indirect driving system in place of the DD motor driving system.
As shown in
The gantry stationary section 4 includes a protruding portion, which is substantially S-shaped, for a satisfactory engagement with the concave portion of the gantry rotary section 3, and a bearing or the DD motor 1 is mounted to the protruding portion. The other construction of the gantry stationary section is omitted from the drawing. Also, a simplified encoder 8 for detecting the rotation is mounted to the outside (opposite to the rotary section) of the edge portion of the gantry stationary section 4. The simplified encoder 8, which is a general purpose encoder, is smaller than the resolver, high in reliability and cheap. The simplified encoder 8 has a rotary shaft, and a driven gear 9 meshing with the rotary section gear 10 is mounted to the rotary shaft. It is desirable for the driven gear 9 to be formed of a material slightly softer than the material of the rotary section gear 10 because, in this case, the relatively small driven gear 9, not the relatively large rotary section gear 10, is abraded first and, thus, it suffices to renew mainly the driven gear 9 alone.
In accordance with rotation of the gantry rotary section 3, the driven gear 9 is rotated and the rotary shaft of the simplified encoder 8, which is coaxial with the driven gear 9, is also rotated. As a result, the simplified encoder 8 generates pulse signals conforming with the rotation of the rotary shaft. The pulse signals thus generated are utilized for the detection of the rotating position and the for the detection of the rotating speed of the gantry rotary section 3.
The fourth embodiment described above is more advantageous in cost than the resolver system because the simplified encoder 8 is used in the fourth embodiment. It should also be noted that, if the simplified encoder 8 should be broken, the simplified encoder can be renewed easily by detaching the cover 6 and, thus, the maintenance can be performed easily.
The present invention is not limited to the first to fourth embodiments described above and can be worked in variously modified fashions.
As described above, the present invention provides an X-ray CT apparatus simple in construction, capable of maintenance with a low cost, and capable of the rotation detection or the rotation control with a high accuracy.
Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.
Kuroda, Masahiro, Sasaki, Tomiya, Kasuya, Yuichi, Takanashi, Tetsuyuki
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